Super-conductive magnet systems are used in many of the presently available and proposed NMR diagnostic imaging devices. Liquid helium is used to cool the electro-magnets to the low temperature required. One of the problems with liquid helium cooled systems is that low temperature is maintained by the "Boil Off" of the liquid helium that occurs when its temperature reaches 4.2 Kelvin (at atmospheric pressure). Due to the "Boil Off" the helium has to be replaced at frequent intervals. Thus, costly and bothersome replacement of the helium is required after relatively short time periods.
An apparent solution is to use a liquifier to recompress the boiled off liquid helium. However, such liquifiers are large, unwieldly and expensive units.
A proposed solution is to reduce the amount of liquid helium boiled off by lowering the temperature of a radiation shield surrounding the liquid helium tank.
In theory, such solutions sound promising. However, there are many practical problems involved. The problems have to be solved if there is to be a cryogenic cooler available that can be lowered to a low enough temperature so as to efficiently reduce the temperature difference between the shield and the helium container.